Current and future SoC will contain an increasing number of heterogeneous multiprocessor subsystems combined with a complex communication architecture to meet flexibility, performance and cost constraints. The early validation of such complex MP-SoC architectures is a key enabler to manage this complexity and thus to enhance design productivity. In this paper, we describe an abstract, high level CPU subsystem model that captures the specificities of such MP-SoC architectures, along with a timed co-simulation environment to perform early exploration of the entire HW/SW design. The model is based on the Hardware Abstraction Layer (HAL) concept allowing the validation of complex applications written on top of real-life operating systems. Experimentation with a MPEG4 application proves the interest of the proposed methodology.